Several radionuclides decaying by electron capture (EC) and internal conversion (IC) are widely used for imaging in Nuclear Medicine. Such radionuclides also emit numerous low and very low energy Auger electrons, and may cause extensive damage in biological systems, similar to the case of 125I. It is proposed to study the radiotoxicity of several commonly used radionuclides in vivo in experimental mice. Spermatogenesis will be used as a model to observe any excessive damage caused by the low energy electrons from the incorporated radionuclides. Comparison with the effects of beta-decaying radionuclides of the same element give the relative biological effectiveness (RBE) of the incorporated EC and IC radionuclides. Such information may be useful to estimate better a realistic radiation dose to the germ cells in the gonads of the patient from the clinical procedures. The observed RBE values may be considerably higher than l if highly localized energy deposition by the low energy electrons plays an important role. Completed studies give RBE of about 3.5 for 20lT lambda relative to 204T lambda. To understand the RBE values, autoradiographic and other studies will be conducted to determine the distribution of the radionuclides; theoretical Auger and Coster-Kronig electron spectra will be calculated; and appropriate dosimetric models will be developed. The low energy Auger electrons also offer new possibilities not yet explored. Using radiolabeled DNA-seeking chemotherapeutic agents such as cis-platimum, it is proposed to investigate in mouse tumor models possible enhancement of therapeutic efficacy with reduced chemotoxicity of the drug.